The present invention concerns photovoltaic semiconductor devices. In particular, the present invention is a device whose absorption of incident radiation is enhanced. Examples of photovoltaic semiconductor devices include solar cells, and photodiodes. Most of the following discussion will be directed towards solar cells; however, the general discussion applies to all photovoltaic devices.
Absorption of light in a solar cell is governed by the optical properties of the semiconductor materials used to form the active layers of the cell. Optical absorption in the active layers imposes constraints on the minimum thickness of the semiconductor materials used to form the cell. Such thickness requirements impose severe purity constraints on the semiconductor materials. This follows because the purity of the active material in part determines the lifetimes of the electron hole pairs that are generated by absorbed light. The lifetimes determine the average length or collection width over which carriers can be collected in the device. To maximize this can require high purity active material. Therefore, it is desirable to enhance the effective absorption within the active material because (1) the thickness of the active material can be reduced and (2) semiconductor purity requirements can be relaxed.
These considerations are particularly important for amorphous and indirect gap semiconductors. In indirect gap semiconductors, like silicon, the solar radiation is weakly absorbed. For silicon more than 50 microns of material is required to effectively absorb solar radiation. If the optical absorption were enhanced, thinner cell thickness would be required and lower purity material could be used. In amorphous silicon, blue light is absorbed in a thickness of less than 1 .mu.m while near infrared radiation (750-800 nm) requires at least 10 .mu.m for complete absorption. Since the cell must have a minimum thickness to allow for absorption of incident sunlight, the collection width must be of the order of the cell thickness to allow for the generated electron-hole pairs to contribute to the electric current. Since the collection width for amorphous silicon is at best 1.5 .mu.m, optical enhancement could provide a significant improvement in the collection efficiency of near infrared radiation. This in turn should allow the overall cell efficiency to be increased.
In the past decade, there have been a number of suggestions for the use of light trapping by total internal reflection to increase the effective absorption in the indirect gap semiconductor, crystalline silicon. The original suggestions (A. E. St. John, U.S. Pat. No. 3,487,223 and O. Krumpholz and S. Moslowski, Z. Angew. Phys. 25, 156 (1968)) were motivated by the prospect of increasing the response speed of silicon photodiodes while maintaining high quantum efficiency in the near infrared.
Subsequently, it was suggested (D. Redfield, App. Phys. Lett. 25, 647 (1974) and D. Redfield, U.S. Pat. No. 3,973,994) that light trapping would have important benefits for solar cells as well. High efficiency could be maintained while reducing the thickness of semiconductor material required. Additionally, the constraints on the quality of the silicon could be relaxed since the diffusion length of minority carriers could be reduced proportionate to the degree of intensity enhancement. With such important advantages, interest in this approach has continued, but no significant advances in the design of optically enhanced solar cells have been made.